c++ - 如何在专门化交换功能时使用 gcc8 修复编译错误/

Question

我正在尝试为我的类编译专门化函数 std::swap 的代码。但是我遇到了由于右值构造函数而发生的问题(当我评论该行时它有效)

我正在使用 g++ (GCC) 8.2.1 20180905 (Red Hat 8.2.1-3) 进行编译和 g++ -std=c++14 -Wall 那些选项

#include <iostream>
#include <utility>

namespace ns1
{
class Foo 
{
public:
    Foo();
    Foo(const Foo& that) {};
    Foo(Foo&& that) {};  // <--- work when commented
    ~Foo();  
    void swap(Foo &that) {};
};

inline void swap(Foo &lhs, Foo &rhs)
{
   lhs.swap(rhs);
}
}  // namespace ns1

namespace std {

template <>
inline void swap(ns1::Foo &lhs, ns1::Foo &rhs)
{
   lhs.swap(rhs);
}

} // namespace std

我有以下错误信息:

 error: template-id 'swap<>' for 'void std::swap(ns1::Foo&, ns1::Foo&)' does not match any template declaration
 inline void swap(ns1::Foo &lhs, ns1::Foo &rhs)
             ^~~~
In file included from /opt/rh/devtoolset-8/root/usr/include/c++/8/string:52,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/locale_classes.h:40,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/ios_base.h:41,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/ios:42,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/ostream:38,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/iostream:39,
                 from toto.cpp:1:
/opt/rh/devtoolset-8/root/usr/include/c++/8/bits/basic_string.h:6276:5: note: candidates are: 'template<class _CharT, class _Traits, class _Alloc> void std::swap(std::basic_string<_CharT, _Traits, _Alloc>&, std::basic_string<_CharT, _Traits, _Alloc>&)'
     swap(basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~
In file included from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/stl_algobase.h:64,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/char_traits.h:39,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/ios:40,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/ostream:38,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/iostream:39,
                 from toto.cpp:1:

Answer 1

您看到的错误是因为提供用户定义的移动构造函数会阻止编译器合成移动赋值运算符，从而使您的类不可移动赋值。根据 cppreferece , swap 中的 T 必须是可移动赋值的。

要修复它，请同时提供移动赋值运算符，demo

Foo& operator=(Foo&& other) {return *this;}
Foo& operator= (const Foo& other) { return *this; }

另请查看 rule of five